The present invention relates generally to electrostatographic reproducing apparatus and in particular to a developing apparatus and methods for use in such a machine. More particularly, the present invention is directed to an improved screen donor member and method of using same in touchdown development.
Generally in the process of electrostatographic printing a photoconductive insulating member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive insulating layer is thereafter exposed to a light image of an original document to be reproduced. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After recording the electrostatic latent image on the photoconductive member, the latent image is developed by bringing a developer material charged of an opposite polarity into contact therewith. Toner particles are attracted to the electrostatic latent image to form a toner powder image which is subsequently transferred to a copy sheet and thereafter permenantly affixed to the copy sheet by fusing thereto.
The development of the electrostatic latent images may be carried out in a variety of ways. Development systems well known and developed in the prior art include those described in U.S. Pat. No. 3,618,552 Cascade Development; U.S. Pat. Nos. 2,874,063, and 3,251,706 and 3,357,402 Magnetic Brush Development; U.S. Pat. No. 2,217,776 Powder Cloud Development and U.S. Pat. No. 3,166,432 Touchdown Development.
Transfer development broadly involves bringing a layer of toner to an imaged photoconductive insulating layer where toner particles will be transferred from the layer to the imaged areas. In one touchdown development technique, a layer of charged toner particles is applied to a donor member which is capable of retaining the particles on its surface and then the donor member is brought into close proximity to the surface of the photoconductor. In the closely spaced position particles of toner in the toner layer on the donor member are attracted to the photoconductor by the electrostatic charge on the photoconductor opposite to the toner charge so that development takes place. In this technique the toner particles must traverse an air gap to reach imaged regions of the photoconductor. In the other touchdown development techniques the toner laden donor actually contacts the imaged photoreceptor and no gap is involved. In one such technique the toner laden doner is rolled in non-slip relationship into and out of contact with the electrostatic latent image to develop the image in a single rapid step. In another such technique the toner laden donor is skidded across the xerographic surface. Skidding the toner by as much as the width of a thin line will double the amount of toner available for development of the line if it lies perpendicular to the skid direction. The amount of skidding can be increased to achieve greater density or greater area coverage.
In a typical transfer development system, a cylindrical or endless donor member is rotated so that its surface can be presented to the moving surface of a photoconductive drum bearing an electrostatic latent image thereon. Positioned about the periphery of the donor member are a number of processing stations including, a donor loading station, at which toner is presented to and coated on the donor member surface; an agglomerate removal station at which toner agglomerates and excess toner are removed from the toner layer retained on the surface of the donor member; a charging station at which a uniform charge is placed on the particles of toner retained on the donor surface; a clean up station at which the toner layer is converted into one of uniform thickness and uniform charge state at which any toner agglomerates not removed by the agglomerate removal station are removed; a development station at which toner particles carried by said donor member are presented to the imaged photoconductor for image development; and a cleaning station at which a neutralizing charge is placed upon the residual toner particles and at which a cleaning member removes residual toner from the peripheral surface of the photoreceptor. In this manner, a continuous development process is carried out.
Among the donor members employed in the prior art are those embodying the principles described in U.S. Pat. No., 3,203,394. Such a donor includes, an electrically conductive support member in the form of a cylinder, a thin electrically insulating layer overlying a support member, and a continuous, electrically conductive screen pattern is provided with an electrical connection to a slip ring so that its potential may be varied between ground potential and a charge potential at different stages of process. A multitude of high fringe fields or microfields are created at the surface of this type of donor member. When this type of donor member is brought into contact with toner particles it is loaded with toner.
A donor member of this type is quite expensive to manufacture, it is quite fragile in the screen regions and is subject to being electrically disabled, e.g., through shorting of the screen to the conductive substrate, unless considerble care is taken during its manufacture and use.
Recently a touchdown development technique which is simplier, has a more reliable donor member and more cost effective has been developed. In the cross referenced copending application, a touchdown development technique is provided for loading a donor member in a simple, uncomplicated process which includes inserting an open mesh screen in a toner loading hopper and directly contacting the toner member in the toner loading zone. The screen serves to friction charge the toner after it passes through the open mesh and rubs against the donor member thereby forming a dense, uniform layer of toner on the surface of the donor member. This system has the advantages of minimizing airborne dust, simplicity, elimination of toner concentration problems, and provides excellent solid area coverage. The donor member described in said copending application Ser. No. 549,096 employed an anodized aluminum roll which initially worked very well in charging toner particles but with continued use on ageing experienced a marked reduction in its capability to charge the toner effectively and efficiently. Furthermore in operating at a relatively high humidity level, the charge level produced on the toner was substantially reduced, frequently reaching levels where unacceptable development could be obtained. In addition the anodized aluminum donor member is hard and since this must run in contact with a photoreceptor surface, the opportunity and frequency of damage to the photoreceptor surface by coming into contact therewith is dramatically increased.